The complement system can be activated through three distinct enzymatic cascades, referred to as the “classical pathway”, “Lectin/MBL”, and “alternative” pathway” (CP, MBL, and AP respectively). MBL is not discussed here. The classical pathway is responsible for aiding in host defense against antigens to prevent infection of cells. The lectin pathway is a variation of the classical pathway. The alternative pathway is currently thought to be responsible for 80-95% of total complement activity in cases where trigger of complement activation is the classical pathway (“AP amplification loop”). The alternative pathway by itself is activated in a number of disease indications where complement components have been found in elevated state.
There are three “alternative pathway specific proteins”; Factors B, D, and P, which play a major role in the; a) initiation and propagation of the alternative pathway and b) classical pathway propagation via the alternative pathway amplification loop. Proteins C3 and C3b, the key players in complement system, are common to all classical and alternative complement pathways. While there may be a one type of C3, there are three different types of C3b produced as each C3 convertase is different. AP C3 convertase is composed of PC3bBb, the classical C3 convertase is made up of different proteins. Therefore it is hard to believe that the cut would be all identical to produce similar C3b molecules. As a result, C3b produced by the alternative pathway is different compared to C3b produced via the classical pathway.
The classical pathway (CP) is initiated by antigen-antibody complex. The CP progression involves proteins such as C1Q, C1r/C1s, C4, and C2. The CP C3 convertase consists of C3bC4b2a. This complex can cleave the C3 into C3b and C3a. This C3b is derived from classical pathway convertase and is usually required for opsonization of various pathogens and bacteria. Inhibition of this C3b is undesirable. C3b coated cells are removed via complement receptors present on various cells.
Both complement pathways independently produce C3a, C3b, C5a, C5b, C5b-9, and sC5b-9 as complement activation byproducts.
During classical pathway triggered activation of the alternative pathway, Classical pathway C3 convertase also cleaves C3 into C3b which can work independent of the alternative pathway with full amplification of the classical pathway in 1% normal human serum in the presence of Ca2+/Mg2+ ions. Classical pathway C5 convertase can cleave C5 to generate C5a and C5b. The C5b molecule then inserts into the lipid bilayer of the cell to initiate the formation of C5b-9 or sC5b-9.
In alternative pathway activation, C3b produced by the complement system can bind properdin and Factor B to form the complex “PC3bB”. Factor D then cleaves Factor B, within the complex, into Bb and Ba. This cleavage results in the release of Ba from the complex and the formation of the AP convertase PC3bBb. PC3bBb cleaves C3 into C3a and C3b, thereby perpetuating the amplification loop of the alternative pathway for the benefit of the alternative pathway. PC3bBb can then cleave C5 to make C5b and C5a. The C5b molecule then inserts into a lipid bilayer of a cell and forms the nucleus for MAC deposition.
The classical pathway can also initiate the propagation of a part of the alternative pathway known as the amplification loop. Within the amplification loop, C3b binds properdin and Factor B to form the complex “PC3bB”. Factor D then cleaves Factor B, within the complex, into Bb and Ba. This cleavage results in the release of Ba from the complex and the formation of the AP convertase PC3bBb. PC3bBb cleaves C3 into C3a and C3b, thereby perpetuating the amplification loop.
C3b is therefore both a component and a byproduct of the complement system irrespective of the type of complement pathway activation. During the amplification of the AP, as the PC3bBb (AP C3 Convertase) generates increasing amounts of C3b, an amplification loop is established so that activation of the alternative pathway can continue. Furthermore, the classical pathway can also generate C3b, which can bind factor B and thereby engage the alternative pathway, even though the trigger is CP mediated. This allows more C3b to deposit on a target, which leads to enhanced amplification of AP activation.
Addition of newly formed C3b to the existing AP C3 convertase PC3bBb generates the AP C5 convertase. Addition of newly formed C3b to the existing CP C3 convertase generates CP C5 convertase. Both C5 convertases have the ability to cleave C5 to produce C5b and C5a. The terminal complex produced as a result of complement activation is known as the MAC complex (also known as C5b-9 or sC5b-9), which is responsible for lysis of cells in a subject. Both C3a and C5a are potent anaphylatoxins that are responsible for activating platelets, neutrophils, and monocytes. As a result, inflammatory molecules such as elastase, TNF-α, IL-1, VEGF, and peroxides are released. Formation of C5b-9/sC5b-9 is responsible for tissue damage and tissue injury/tissue damage seen in “other diseases”
Classical complement pathway activation provides a valuable first-line defense against potential pathogens and can generate C3a/C3b, C5a/C5b, and C5b-9/sC5b-9. Therefore, exacerbation of the classical pathway can produce large amounts of complement byproducts. As described elsewhere, both C3a and C5a are potent anaphylatoxins, C3b mediates opsonization, and C5b is responsible for wanted killing of the pathogens. Here, both C3a and C5a would generate beneficial responses and are produced to kill the invaders. This pathway is required for host defense and therefore must not be inhibited.
Alternative pathway activation in Mg++ ions, without the calcium ions, guarantees only the AP activation. In disease state, this pathway is activated independent of the classical pathway. This pathway is not required for host defense and therefore can be inhibited in its entirety.